This invention relates to power supply circuits for use in portable electrophotographic apparatus and more particularly, to a portable, modular corona power supply circuit of the high-voltage low-current type for use in such apparatus.
In the field of xerographic or electrostatic image recording, a latent image is formed on a plate having a photoconductive surface. This latent image is the pattern which is projected onto the plate by a lens system or other suitable means. Prior to projection, the plate is subjected to a corona discharge to provide a large number of electrons at or near the photo-conductive surface which are capable of migrating when bombarded by photons.
An apparatus for processing images utilizing electrophotographic film is described in detail in the copending applications. The electrophotographic film is comprised of a substrate of polyester or the like carrying an ohmic layer bonded thereto and an inorganic photoconductive coating is bonded to the surface of the ohmic layer. The film is exposed to a corona discharge which is formed when an electrode is subjected to a voltage on the order of 6000 volts; the electrode being adjacent said film.
The corona discharge results in a charge of electrons on or near the surface of the photoconductive coating; exposure to a light pattern causes a patterned discharge of the electrons to produce a latent image; the application of toner particles to the surface results in a variegated adherence of particles. Thus, a visible image is formed by the patterned adhesion of toner particles to the surface of the photoconductor.
Given the necessity of building a small handheld camera type of device for producing the electrostatic record, a small low power consumption power supply circuit of the type capable of supplying corona voltages also would be required. Such prior art supplies were complex, bulky, expensive and not easily adaptable for portable use as would be required in a camera of the above type. So far as known, prior corona voltage circuits continuously were connected to the power source, making them difficult to use with low energy sources, such as for example dry cell batteries. Those which may not have been continuously connected employed oscillator power supplies which made no attempt at minimizing current drain or power consumption.
Prior art power supply circuits overstressed the active elements within the power supply causing component failure and/or break down or "run away" of the circuit when the corona load at the output of the power supply varied from a load to a "no load" condition.
In toning prior electrostatic plates or members, the toner tends to migrate to the edges of areas which were fully charged so that an edge effect occurs. According to the disclosures of the copending application Ser. No. 323,108, a D.C. bias is utilized to drive the toner particles to the photoconductive surface and provide for uniform distribution thereof over the surface. This obviates edge effect. The circuit of the invention herein also provides the positive voltage to be used for such bias.